In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this SNF is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, SNF is first placed in a canister, which is typically a hermetically sealed canister that creates a confinement boundary about the SNF. The loaded canister is then transported and stored in a large cylindrical container called a cask. Generally, a transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store SNF for a determined period of time.
In a typical nuclear power plant, an open empty canister is first placed in an open transfer cask. The transfer cask and empty canister are then submerged in a pool of water. SNF is loaded into the canister while the canister and transfer cask remain submerged in the pool of water. Once the canister is fully loaded with SNF, a lid is placed atop the canister while in the pool. The transfer cask and canister are then removed from the pool of water. Once out of the water, the lid of the canister is welded to the canister body and a cask lid is then installed on the transfer cask. The canister is then dewatered and backfilled lied with an inert gas. The transfer cask (which is holding the loaded canister) is then transported to a location where a storage cask is located. The loaded canister is then transferred from the transfer cask to the storage cask for long term storage. During transfer of the canister from the transfer cask to the storage cask, it is imperative that the loaded canister is not exposed to the environment.
One type of storage cask is a ventilated vertical overpack (“VVO”). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. Traditional VVOs stand above ground and are typically cylindrical in shape and are extremely heavy, often weighing over 150 tons and having a height greater than 16 feet. VVOs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of SNF, and a removable top lid.
In using a VVO to store SNF, a canister loaded with SNF is placed in the cavity of the cylindrical body of the VVO. Because the SNF is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that this heat energy have a means to escape from the VVO cavity. This heat energy is removed from the outside surface of the canister by ventilating the VVO cavity. In ventilating the VVO cavity, cool air enters the VVO chamber through bottom ventilation ducts, flows upward past the loaded canister as it is warmed from the heat emanating from the canister, and exits the VVO at an elevated temperature through top ventilation ducts. Such VVOs do not require the use of equipment to force the air flow through the VVO. Rather, these VVOs are passive cooling systems as they use the natural air flow induced by the heated air to rise within the VVO (also know as the chimney effect).
While it is necessary that the VVO cavity be vented so that heat can escape from the canister, it is also imperative that the VVO provide adequate radiation shielding and that the SNF not be directly exposed to the external environment. The inlet duct located near the bottom of the overpack is a particularly vulnerable source of radiation exposure to security and surveillance personnel who, in order to monitor the loaded VVOs, must place themselves in close vicinity of the ducts for short durations. Therefore, when a typical VVO is used to store a canister of SNF in its internal cavity, the canister is supported in the cavity so that the bottom surface of the canister is higher than the top of inlet ventilation ducts. This is often accomplished by providing support blocks on the floor of the cavity. By positioning the bottom surface of the canister above the inlet ventilation ducts, a line of sight does not exist from the canister to the external atmosphere through the inlet ventilation ducts, thus eliminating the danger of radiation shine out of inlet ventilation ducts. However, as discussed below, positioning a canister in the cavity of a VVO so that the bottom surface of the canister is above the top of the inlet ventilation ducts creates two issues: (1) a potential cooling problem during a “smart flood” condition; and (2) an increased height of the VVO.
Subpart K of 10 C.F.R. §72 provides for a “general certification” of casks for on-site storage of SNF. A number of casks have been licensed by the United States Nuclear Regulatory Committee (“U.S.N.R.C.”) and are listed in subpart L of 10 C.F.R. §72. These casks are certified to store a whole class of SNF (including SNF coming from pressurized water reactors (PWRs) or boiling water reactors (BWRs)). Unfortunately, reactors burn fuel in a wide variety of lengths. For example, PWRs in the U.S. presently burn fuel as short as 146″ (e.g., Ft. Calhoun) and as long as 198″ (e.g., South Texas). A general certified cask has been licensed in one or two fixed lengths (models) by the U.S.N.R.C. However, if the SNF is too long to fit in a licensed cask, then the cask simply cannot be used. Moreover, if the SNF is too short, then axial spacers are used to fill the open space in the storage cells to limit the movement of SNF in the axial direction. Thus, most casks and canisters used in the on-site storage of SNF have significant open spaces in their storage cells. This condition is particularly undesirable for VVOs because of the adverse consequence to the occupational dose to the plant personnel and cost (because of physical modifications forced on the plant), as set forth below.
First, the dose received by the workers performing the loading operations is directly influenced by the amount of shielding material per unit length in the body of the cask. The total quantity of shielding that can be installed in a transfer cask is governed by the lifting capacity of the plant's cask crane. A longer than necessary transfer cask means less shielding per unit length installed in the cask which in turn results in increased dose to the workers.
In VVOs, the VVO is often loaded inside the plant's truck bay by stacking the transfer cask over the VVO. Minimizing the height of the VVO's body is essential to allow the VVO to be moved out through the plant's truck bay (typically, a roll-up door) after the canister is installed therein. The loaded VVO is typically moved out across the roll-up door without its lid, and the lid is then installed on it immediately after the VVO body clears the door. Therefore, a key objective in the storage VVO design is to minimize the height of VVO body.
In another variation, the transfer cask itself is taken outside through the plant's truck bay and carried over to a pit where the transfer of the canister to the VVO takes place. In this case, the height of the transfer cask must be short enough to clear the plant's roll-up door to avoid the need to shorten the transfer cask (or alternatively, to increase the height of the roll-up door). Shortening the transfer cask is not always possible.